Rational Method and Peak Discharge Lecture Notes

Flashcards covering the fundamental concepts, formulas, and variables associated with the Rational Method for calculating peak discharge in civil engineering and environmental planning.

Peak Discharge

The maximum, highest volumetric rate of water flow passing a specific point in a stream, river, or drainage system during a specific event, representing the top point on a flood hydrograph.

Rational Formula

$Q_p = CiA$ (or $Q_p = CiA k_c$), where $Q_p$ is peak flow, $C$ is the runoff coefficient, $i$ is rainfall intensity, and $A$ is the catchment area.

Drainage/Catchment Area (A)

The total area of all surfaces (roofs, roads, sidewalks, lawns, etc.) draining to one specific outlet, such as an inlet, swale, or culvert.

Watershed Divide

The boundary of a drainage area identified by tracing ridgelines on topographic maps or contours.

Rainfall Intensity (i)

The rate of precipitation, specifically the depth of rainfall falling per unit of time, typically measured in $in/hr$ or $mm/hr$.

Runoff Coefficient (C)

A dimensionless factor representing the portion of rainfall that becomes surface runoff, with higher values for impermeable surfaces like pavement and lower values for permeable areas like forests.

Time of Concentration ($T_c$)

The time required for runoff to travel from the hydraulically most remote point of a watershed to the outlet, indicating when the entire watershed begins contributing flow.

Sheet Flow ($T_{sheet}$)

A segment of flow where runoff moves as a thin layer over the surface during the initial stage of flow.

Shallow Concentrated Flow ($T_{shallow}$)

A segment of flow where water begins to concentrate in small channels or rills.

Channel Flow ($T_{channel}$)

Flow occurrng in defined channels such as streams, gutters, or pipes.

Steel Formula

A formula used to approximate rainfall intensity ($i$) based on the Time of Concentration ($t_c$) using constants $K$ and $b$ specific to a region's climate and return period.

Weighted Runoff Coefficient ($C_w$)

The overall runoff coefficient for nonhomogeneous areas calculated as a weighted average based on the area of each different land use.

Homogeneous Subareas

Smaller divisions of a watershed created to ensure similar slope, soil type, and land cover for more accurate discharge computation.

Hydrograph-based methods

Methods recommended for use when a drainage area contains significant storage (like detention ponds or large channels) that the Rational Method cannot account for.